In just the two years since the release of Apple's iPad, the U.S. tablet market has reached a "critical mass," with nearly one in four smartphone owners also using a tablet in the three-month period ending in April, according to data released by market researcher ComScore.

Tablet use among smartphone owners has more than doubled in the past year, going from 9.7 percent last year to 23.6 percent this year, ComScore found. By comparison, only 10.4 percent of feature phone owners also use a tablet, "suggesting that smartphone ownership is highly predictive of tablet adoption in the current market," comScore said.

"Tablets are one of the most rapidly adopted consumer technologies in history and are poised to fundamentally disrupt the way people engage with the digital world both on-the-go and perhaps most notably, in the home," Mark Donovan, ComScore's senior vice president of mobile, said in a statement announcing the findings. "It's not surprising to see that once consumers get their hands on their first tablet, they are using them for any number of media habits including TV viewing."

Noting that "larger screen sizes [are] making tablets more conducive to video consumption than their smaller-screen cousins," researchers found that tablet users were nearly three times more likely to watch video on their devices compared with smartphone owners, with 9.5 percent of tablet users viewing video content on their device almost daily, the study found.

In terms of demographics, tablet users tend to be older, with the majority of users falling in the 25-to-54 age groups, and wealthier, with the majority of users earning $75,000 a year or more.